The invention relates to a soldering apparatus and a process for soldering printed circuit boards according to the preamble of claims 1 and 18.
The development of electronics in recent years has led not only to a distinct increase in the integration density of semiconductor components, but has also placed new requirements on circuit board production. In particular, the use of novel components (for example BGA), new surfaces (for example Ni-Pd terminals) and the reduction in size of the terminals themselves (fine pitch) leads to the increasing use of inert gases during soldering. Using the latter, process windows can be enlarged adequately, and the quality of the soldered joints meets the increased requirements.
Since in the meantime the focus of printed circuit board production has been transferred to the reflow technique (SMDs), flow-soldering systems are found in standby over relatively long periods of time (a few hours). During this time, a layer of scale forms on the surface of the solder container as a result of oxidation, from which layer, in the absence of further measures, scale particles are entrained by the solder wave during soldering, and can get onto the printed circuit boards. For this reason, solder containers of such flow-soldering systems are encased, and a protective-gas atmosphere is produced in the casing using a non-oxidizing gas, which effectively suppresses the formation of scale.
In addition, it is often necessary, even when soldering with these earlier flow-soldering systems, to achieve low residual oxygen values in the soldering atmosphere, since in this way the soldering result is distinctly improved; at residual oxygen contents xe2x89xa61%, the number of solder beads is distinctly reduced.
Since very few flow-soldering systems are provided for the use of protective gases, casings and gas distribution systems would be desirable, with which the existing flow-soldering systems could be re-equipped.
To this end, a number of hood or tunnel constructions have already been proposed, but these all satisfy only to some extent the requirements for the re-equipment of existing flow-soldering systems. In DE-U1 8520254 a housing of a soldering apparatus is described which is not suitable for the re-equipment of existing flow-soldering systems, since the expenditure for subsequent rebuilding is much too high. Casings for the re-equipment of existing flow-soldering systems in which printed circuit boards are soldered are described in DE 41 42 436 A1 and EP 500 135 B1. In this case, the hoods dip at least partially into the solder bath, with the result that assembly and disassembly, for example for cleaning the solder container or in the event of repairs to the solder container or to the solder wave prove to be difficult in practice. Furthermore, any detachment of the metal of the casing dipping into the solder bath can impair the soldering operation or the soldering quality. In EP 500 135 B1, by designing the casing as a hood, which closes off only the soldering pan itself but not the supply or discharge of the printed circuit boards, it is possible to achieve adequately low residual oxygen values at all only with a very high inert-gas consumption. As a result of the opening of the soldering zone on the entry of printed circuit boards, a considerable quantity of ambient air also passes into the region of the solder bath, which leads to relatively high residual oxygen contents precisely during the soldering operation.
Furthermore, in EP 500 135 B1 only the region of the solder wave is enclosed by the hood. The devices and/or elements reaching into the solder bath, for example the pumps, are only inadequately provided with a protective-gas atmosphere. Large regions remain exposed to the ambient atmosphere, which makes oxidation of the solder bath possible, and hence leads to the formation of a scale layer.
The invention is based on the object of providing a casing for a soldering apparatus, by means of which the existing flow-soldering systems can be simply and cost-effectively re-equipped.
Starting from the prior art cited in the preamble of claim 1, this object is achieved with the features specified in claim 1.
Advantageous developments are specified in the subclaims.
According to the invention, the casing is designed as a modular construction kit having subassemblies and/or constructional elements, preferably comprising an input tunnel, an entry tunnel, at least two covering elements, in each case a covering element for the region of the solder container that accommodates the printed circuit boards and the solder wave, and a covering element for the region of the solder container that accommodates the devices and/or elements, the two covering elements having a connecting point that runs in the transport direction of the printed circuit boards and together forming a hood which covers the entire solder container and has a projecting apron, an outlet tunnel and detachable connecting means on the solder container and/or the subassemblies and/or constructional elements to connect the subassemblies and/or constructional elements and the solder container. As a result of the design according to the invention of the casing from modular subassemblies/constructional elements, which are joined with low effort at specially designed connecting points, assembly and disassembly at the customer are reduced considerably. Since the modular subassemblies/constructional elements are simply constructed, on the one hand the production is cost-effective and on the other hand high flexibility is ensured, since each subassembly/constructional part can simply be adapted at the customer to the existing flow-soldering system. In this case, the modular construction kit is configured in such a way that it ensures a high degree of sealing and hence low residual oxygen values in the protective-gas atmosphere within the casing are possible. In the case of re-equipping with the casing, the consequent advantage for the customer is that it can rapidly be put to use, since the casing is joined by connecting the modular subassemblies/constructional elements at the specially designed connecting points. Faulty connecting points on the prefabricated modular subassemblies/constructional elements are in this case ruled out. Added to this is the fact that the gas supply for the introduction and distribution of the protective gas in the casing is also modularly constructed, and hence the individual subassemblies/constructional elements are connected to each other in such a way that it is possible to make the existing flow-soldering systems inert with the lowest possible protective-gas consumption. A further reduction in the protective-gas consumption, whilst simultaneously ensuring low residual oxygen contents in the protective-gas atmosphere, is achieved by controlling the gas distribution.